In integrated circuits including CMOS integrated circuits, it is often desirable to be able to permanently store information, or to form permanent connections on the integrated circuit after it is manufactured. Fuses or devices forming fusible links are frequently used for this purpose. Fuses can be used to program redundant elements to replace identical defective elements, for example. Fuses can also be used to store die identification or other such information, or to adjust the speed of a circuit by adjusting the resistance of the current path.
In some cases, electrically erasable programmable read only memory (EEPROM) devices are used to perform the discretionary connection function of a fuse device. Semiconductor process technologies continue to provide for smaller device geometries and operate at lower voltages. As the device geometries are reduced, so is the thickness of the gate oxide layer. EEPROM fuse devices require a relatively thick gate oxide to prevent high leakage currents and sustain a charge on the floating node. Thus, EEPROM fuse devices are not viable for use on many of the latest semiconductor process technologies.
Other fuse devices require an extra semiconductor processing step to form or program the discretionary connections. For example, one type of fuse device is "programmed" using a laser to open a link after the semiconductor device is processed and passivated. This type of fuse device not only requires an extra processing step to program or "blow" the fuse devices where desired, but also requires precise alignment of the laser on the fuse device to avoid destroying neighboring devices. This and other similar approaches can result in damage to the device passivation layer, and thus, lead to reliability concerns. In some approaches, the passivation layer must actually be removed before programming the fuse device to provide space for the fuse material when the connection is destroyed. In other approaches, where the passivation layer is not intentionally removed, the process of blowing the fuse can cause a hole in the passivation layer when the fuse material is displaced.
Another type of fuse device, referred to as an "oxide antifuse" device, is programmed using high voltages compared to the circuit's normal operating voltage supply. Therefore, the peripheral circuitry for these devices, generally requires higher junction breakdown voltages than those normally available on the integrated circuit. This is an issue for newer process technologies, and those in development, for the same reasons described above in reference to EEPROM fuse devices. The reduced gate oxide thicknesses require higher well doping which results in lower junction breakdown voltages, and thus, oxide antifuse devices may also not be viable for use with many of the latest process technologies.
The invention provides a small fuse device which can be manufactured without additional processing steps, and which can be reliably used with today's lower voltage, thinner gate oxide process technologies. Further, the fuse device of the invention can be programmed using relatively low voltages without damage to overlying dielectric layers.